Incandescent metallic lamp filament



T-A ril 19, 193

c. A. LAISE 2,114,426

INCANDESCENT METALLIC LAMP FILAMENT 7 Filed Oct. 19, 1936 zo/va Jl/VGlECRYSML ZemmsALazlse IINVENTOR BY flout Auk:

, ATTORNEY Patented Apr. 1 938 UNITED STATES PATEN} F INCANDESCENT METALLIC LAMP FILAMENT Clemens A. Laise, Tenafly, N. J. Application October 19; 1036, Serial No. 106,275

'2 Claims. (01. 176-132) This invention relates to improvements in incandescent metallic. lampfilaments and methods of producing such filaments.

The object of the invention is to provide a fila- 5 ment having long life, slight drop in lumen output in the course'of its life and a slight distortion or sag caused by a limited freedom of motion between the crystals of the filament. This will prolong the life of thev filament.

Aswell known, thoriated tungsten filament has, after burning, a fine crystalline structure. Such crystalline structure gives and the filament will withstand vibration over long periods of time.

However, thoriated tungsten filament tends to sag and it has been to a great extent replaced by so-called non-sag tungsten filament formed of very long overlapping or interlocking crystals. Such crystal growth is usually brought about "either by heat treatment of the tungsten from 0 which the filament is made or by supplying silicates or other ben eficial'additionsf to tungsten oxide or tungstic acid. During the initial burning or flash-ageing of the filament the crystals will grow until the average cross-section is occupied by a single crystal. These crystals are usually 00- extensive with one or more turns of a coiled filament and are sometimes several inches long.

Tungsten filaments made up of long, interlocking or overlapping crystals sag very littleor not at all (depending on the length of the crystals) during their normal commercial life. This is a highly desirable feature, particularly where coiled filaments are used as is now generally the practice, but-these non-sag filaments break easily and cannot be used where the lamp is subjected to shocks or vibrations. The susceptibility to breakage is probably due to the long, rigidly interlocked crystals. The crystals of such filaments are not free to move when the filament is burning.

The filament produced in accordance with the present inventionhas a crystalline structure which will resist shocks and vibration to'substantially the same extent as the thoriated filament, but which will sag so little that the drop in lumen output in the course of its life will not be more than from 6 to 12%, but never materially in excess of 10%. It has a longer life than non-sag filaments or the thorlated multi-crystal type especially in coil form. By the well-known hairpin test, the usual sag of my wire is 5-7 mm. at .0075" diameter, whereas the usual sag of theso-called non-sag wire is 1-2 mm. I v Toaccomplish this I purify and wash with dis tilled water a slurry of precipitated tungsten oxide or tungstic acid and then add to it a solution so that afterdrying with constant stirring the oxide will contain (by weight) about 1% KzWO4 (potassium tungstate) and about to /2% of 'NaCl (sodium chloride)- or other alkaline salts.

Lithium tungstate or sodium tungstate; or mix- 5 tures of these with potassium tungstate, may be used. I may slurry these alkaline tungstates into the tungsten powd'er'instead of into the oxide. However, I prefer adding them to purified tungsten oxide and prefer to produce the oxide by precipitating one half from sodium tungstate and producing the other half by igniting ammonium paratungstate. This will produce a mixture of coarse and fine tungsten oxide and the resultant metal produced by reduction with hydrogen will 5 thereforeconsist of a mixture of coarse and fine particles, most desirable for-the purpose of pro ducing my improved filament. Metal containing fine andcoarse particles may also be produced by I adjusting the reduction temperatures and the procedure in the manner well known in the art.

The metal powder from which the ingot is to be produced consists, therefore, of a mixture of fine and somewhat coarser particles of tungsten coated with a thin film of alkaline tungstate or partially reduced alkaline tungstate. This powder is compressed into suitable size ingots and .is then treated at white heat of about 90% of fusing current to cause the particles to sinter together and produce a fine, compact ingot somewhat softer and more 30 pliable than ordinary tungsten and having a grain count not exceeding 5000 grains per square mm. There will be a thin film of alkaline bronzes between the grain boundaries of the crystals. Prob- -my filament is traversed by current without completely obstructing the growth of the crystals during the annealing operation and the final heat 40 treatment. Inthe customary flash-ageing of the lamp, the filament particles will be comparatively quickly brought through the germinating 'temperature and will acquire an almost permanent set, after which the grains will have a slight amount 45 of freedom of motion but only of such a degree that drop in lumen output of the filament coil will not be excessive.

The metallic ingot from which the filament is produced is much softer and more pliable than 50 the ingots used in the production of thoriated,

' silicated, or non-sag tungsten wire. It is, therefore, economical to reduce this ingot to the desired size and this can beaccomplished at lower temperatures than was heretofore practicable. In

the manufacture of my wire a combination of annealing and warm or hot ,drawing may beused. In the annealing operation the particles of metal are recrystallized from the usual fibrous structure at white heat in a reducing or hydrogen atmosphere. Before the wire is permitted to attain room temperature it is drawn througha diamond die or the like. This will elongate the grains below germinating or equi-axing temperature of the metal, but above room temperature, usually at red or dull red heat.

Since the metallic body is much softer and more pliable than was the case with prior tungsten bodies, the drawing of the wire becomes cheaper and this will result in a saving of diamond drawing dies and also in wire rejections. The cheapening of the drawing operation is enhanced also by the possibility of drawing the wire down inlarger steps than has heretofore been practicable on a commercial scale.

Figs. and 6 of the drawing illustrate the crystalline structure of wire made in accordance with my invention as contrasted with prior structures shown in Figs. 1 to 4.

Fig. 1 illustrates the usual thoriated wire after it has been burned for two hours at normal operating voltage;

Fig. 2 illustrates the thoriated wire after burning for 500 hours. It will be seen that the thoriated wire even after 500 hours burning has a fine grain structure, the cleavage planes of the grains being such as to permit sagging of the filament.

Fig. 3 illustrates the usual silicated wire after two hours burning;

Fig. 4 the same wire after 500 hours burning. As shown in the drawing, two very long crystals extending to the full width of the wire interlock and overlap, the linking of the two sections of the wire being so rigid as to prevent all sagging, but also subjecting the wire to the danger of breakage by vibration or shock. v

- My wire is illustrated in Fig. 5 after two hours of burning and in Fig. 6 after 500 hours of burning. -It will be seen that while the crystals are coarser than thoseof the thoriated wire, still they are much smaller than those of the silicated wire and are so interlocked as to prevent objectionable sag. .Since the crystals do not extend to the full widthof-the wire, and since their length is usually less than a complete turn of the usual coil, there will be sufilcient play between adjacent crystals to give when subjected to vibration or shock.

At no point will the cleavage planes between crystals extend to the full width of the filament and perpendicularly to the axis thereof. Thus oflsetting and sagging will be prevented.

Initially, the filaments shown in all the figures have substantially the same kind of fibrous structure.

I have described only those steps in the manufacture of my tungsten filament which are of particular importance in the present case; in other respects the production of the ingot, the drawing of the wire and the assembling of the lamp follows the well-known procedure.

Obviously, the method described may be varied without departing from the spirit of the invention, certain features of which are applicable also to wire made of tantalum, rhenium, hafnium and the like and alloys thereof.

The tungsten wire produced by my process has such a fibrous structure that when heated to white incandescence in a lamp, the filament becomes recrystallized into elongated interlocking grains which do not extend across the entire width of the wire. The length of the individual crystals is much greater than their width, but

they are not sufficiently'long to produce a nonsag wire or to extend the whole length of a coil or a turn of a coil; The sag of my improved filament'ls such that it is not less than 5 mm. (by the hairpin test). At normal burning the commercial life of the filament will be up to 30% longer than that of lamps equipped with the usual non-sag filaments, and the drop in lumen output after 500 hours burning will not exceed What is claimed is:

i. A tungsten ingot having a' grain count not exceeding 5000 grains per square mm., and having a thin film of alkaline bronzes between the grain boundaries of the crystals.

2. A tungsten wire having in its grain boundaries traces of tungsten bronzes sufllcient to direct the growth of the crystals when the wire is heated to incandescence in a longitudinal direction and having sufllcient granular cement to retard their growth, so that the crystals produced will be sumciently pliable to permit a lumen drop of not more than 6 to 12% of the normal useful life of the lamp.

CLEMENS A. LAISE. 

